1. Field of the Invention
This invention relates to a method of fabricating a semiconductor device, and in more particular to a method of forming wiring for connecting elements on a semiconductor device.
2. Description of the Prior Art
After elements such as transistors and capacitors have been formed on a semiconductor substrate, the elements are connected to each other by forming wiring, and generally aluminum has been used due to the fact that it is easy to process. However, recently, gold has been used in place of aluminum because of problems with reliability due to electromigration. With gold it is difficult to form patterns using etching and so a plating process as shown in FIGS. 1A to 1C is used.
First, after the elements have been formed on the semiconductor substrate 1, barrier films are formed on the substrate 1 using sputtering. Here a TiW (titanium. tungsten) film 2 and Au (gold) film 3 are formed. The surface of the semiconductor substrate 1 with these barrier films formed on it is normally an insulated surface. Next, a photoresist pattern 5 is formed everywhere except where the wiring is to be formed (see FIG. 1A). This photoresist pattern 5 is used as a mask and a gold plating film 6 is grown using an electroplating method. At this time, the sputtered TiW film 2 and Au film 3 are used as electrical current paths for the plating (see FIG. 1B). Finally, the photoresist 5 is removed chemically, and the gold plating film 6 is used as a mask and the sputtered Au film 3 and TiW film 2 are etched and removed by a dry process such as magnetron ion etching, or are etched and removed by a wet process using aqua regia or hydrogen peroxide (see FIG. 1C).
In this prior plating process, because the photoresist pattern is used as a mask during plating, part of the photoresist is dissolved in a plating solution. The quality of the plating solution is lowered by the dissolved photoresist, and deformities occur due to localized changes in the growing speed of the gold plating film, therefore it is necessary to increase the frequency of replacing the plating solution.
Also, positive type photoresist cracks easily in an alkaline plating solution and so the plating solution seeps into the cracks causing short-circuits to occur between the patterns.
Furthermore, it is easy for a forward taper to occur on the sides of the photoresist pattern, and so when the gold plating film is grown, it is easy for a backward taper to occur on the sides of the gold plating pattern. This becomes very evident from post baking performed before plating. Therefore, when the sputtered Au and sputtered TiW films are removed using the magnetron ion etching, the etched material adheres again in the backward tapered portion and is stripped away during post processing making it easy for short circuits to occur in the patterns.
When the sputtered Au and sputtered TiW films are removed using the wet process, since these sputtered films are used as electrical current paths for the plating, they can be removed only after gold plating has been performed, and so there is a problem during wet etching, in that sideways etching or undercutting occurs below the gold plating film.